1. Technical Field
The invention is related to digital communication devices that both transmit and receive, such as a computer modulator-demodulator (modem) or a network interface circuit for a personal computer, and more specifically to the reduction of near-end cross-talk and echo in such devices.
2. Background Art
A digital communication device useful, for example, in linking a personal computer to a local area network typically must be able to transmit data as well as receive data. In a local area network, the communication device is part of a network interface card of the personal computer, the network being formed by multi-conductor cables connected between the network interface cards of the different personal computers in the network. Typically, a network interface card transmits communications on one set of pins or conductors of the multi-conductor cable while receiving communications on another set of pins of the cable. However, due to mutual coupling between the different conductors of the cable, the signal transmitted by the transmitter portion of the network interface card (the “near end”) may be sensed by the receiver portion of the network interface card along with a signal received from another computer in the network (at the “far end”). This causes interference and is often referred to as near end cross-talk because some portion of the near end transmitter is coupled into the near end receiver. Mutual coupling from the near end transmitter of an adjacent channel is referred to as “near end cross-talk” while reflections from near end transmitter of the same channel is referred to as “echo”. Ideally, only the far end transmitter is seen at the near end receiver. Near end cross-talk and echo can disrupt communications by making it difficult or impossible for the receiver to discriminate the received signal from noise produced by cross-talk and echo from the near end transmitters.
Such cross-talk and echo can be reduced by cancellation. The version of the near end transmitted signal that is actually coupled to the receiver is approximately determined, and its approximate inverse is generated accordingly and applied to the receiver's input as a correction signal. However, it is not possible to predict what portion, if any, of the near end transmitted signal will be coupled to the receiver at any given moment and therefore it is not possible to predict what the correction signal should be. However, the correction signal can be derived using feedback to evaluate the errors of successive attempts and improve the correction signal. For example, well-known gradient descent methods and the like can be employed. Due to the nature of the mutual coupling that causes near end cross-talk, the correction signal may have to include a number of components of the transmitted signal each with a different time delay and a different amplitude, but with a relatively fixed bandpass filter. The delays and amplitudes tend to change over time due to the random nature of the near end cross-talk. Thus, near end cross-talk and echo cancellation is a relatively complex process and requires a significant amount of circuitry on the integrated circuit controlling the network interface.
In the Gigabit Ethernet standard link, there are four separate bi-directional communication channels, each channel having both a transmitter and a receiver at each one of its two ends. At one end of the link (e.g., the near end), therefore, there are four receivers and four transmitters. Each of the four receivers must have a cross-talk or echo canceller for each of the four near end transmitters, yielding sixteen cancellers in all. This represents a great burden in terms of circuit complexity on the integrated circuit, representing a potential trade-off in cost and reliability, a significant problem.